1. Field of the Invention
The present invention generally relates to an image forming method, an image processing method and an image processing apparatus using an error diffusion method in multi-level quantization for multi-tone images.
2. Description of the Related Art
For example, in laser printers, digital copiers, display devices, and other various image processing apparatuses, an error diffusion method has been widely used for falsely reproduicng tones of multi-tone images.
With regard to the error diffusion method, the following various improvements have been proposed:
(1) In a system in which image data is two-level quantized according to the error diffusion method, in order to improve continuity of characters and figures, a configuration (continuity) of ON dots output in the periphery of a target pixel is examined, and a quantization threshold is controlled so that an output dot of the target pixel is easily to generate an ON dot when ON dots continue in a certain direction (Japanese Patent Publication No. 8-24338);
(2) In a system in which image data is two-level quantized according to the error diffusion method, in order to prevent a peculiar texture due to close placement of ON dots due to error diffusion in a positive direction from occurring in a light shade portion, a quantization threshold is controlled so that an output dot of a target pixel is determined to be an OFF dot when ON dots are output in the periphery of the target dot in a light shade portion (Japanese Patent No. 2662402); and
(3) Continuity of output dots in the periphery of a target pixel is examined, and a quantization threshold is controlled so that continuity of ON dots or OFF dots is avoided so that chain-like textures and false contours are prevented from being generated (Japanese Patent No. 2756308).